Scientific Investigations Report 2007–5251
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5251
Version 2.0, June 2013
Ground water has been a major source of agricultural, municipal, and domestic water supply in the Coachella Valley since the early 1920s. Ground-water levels declined throughout the Coachella Valley from the 1920s until 1949. In 1949, the importation of surface water from the Colorado River to the southern Coachella Valley began, resulting in decreased pumping and a recovery of water levels in some areas. Since the 1970s, the demand for water in the southern Coachella Valley has exceeded the deliveries of the imported surface water, and water levels have again declined. The declining water levels have the potential to induce or renew land subsidence in the Coachella Valley. Results of previous studies by the U.S. Geological Survey indicate that land subsidence may have been as much as about 150 mm (0.5 ft) in the southern parts of the valley between about 1930 and 1996 (Ikehara and others, 1997).
The location and magnitude of vertical land-surface changes during 1996–2005 were determined using GPS and InSAR techniques. The GPS measurements and the images processed for the InSAR measurements described in this report span the area from Palm Desert on the north to the Salton Sea on the south. GPS measurements were more useful than InSAR measurements for determining vertical land-surface changes in agricultural areas, and InSAR measurements were more useful for determining spatially detailed changes in nonagricultural areas. GPS measurements made at 13 geodetic monuments in 1996 and 2005 in the southern Coachella Valley indicate that the elevation of the land surface had a net decline of 124 to 9 mm ±54 mm (0.41 to 0.03 ft ±0.18 ft) during the 9-year period. Changes at 9 of the 13 monuments exceeded the maximum expected uncertainty of ±54 mm (±0.18 ft) at the 95-percent confidence level, indicating that subsidence occurred at these monuments between June 1996 and August 2005. GPS measurements made at 20 geodetic monuments in 2000 and in 2005 indicate that the land-surface elevation changed –192 to +51 mm ±36 mm (–0.63 to +0.17 ft ±0.12 ft) during the 5-year period. Changes at 6 of the 20 monuments exceeded the maximum expected uncertainty of ±36 mm (±0.12 ft) at the 95-percent confidence level—subsidence occurred at five monuments and uplift occurred at one monument between August 2000 and August 2005. Data from two of the five subsiding monuments for which subsidence rates could be compared indicate that subsidence rates decreased during this period compared with subsidence rates before 2000.
Results of the InSAR measurements made between May 7, 2003, and September 25, 2005, indicate that land subsided about 75 to 180 mm (0.25 to 0.59 ft) in three areas of the Coachella Valley: near Palm Desert, Indian Wells, and La Quinta; the equivalent subsidence rates range from about 3 to more than 6 mm/month (0.01 to 0.02 ft/month). The subsiding areas near these locations were previously identified using InSAR measurements for 1996–2000, which indicated that land subsided about 35 to 150 mm (0.11 to 0.49 ft) during the four-year period; the equivalent subsidence rates range from about 1 to 3 mm/month (0.003 to 0.01 ft/month). Comparison of the InSAR results indicates that subsidence rates have increased by 2 to 4 times since 2000 in these three areas.
Water-level measurements made at wells near the subsiding GPS monuments and near the areas of subsidence shown by InSAR generally indicate that water levels declined during 1996-2005. The relation between declining water levels and significant subsidence may indicate that the water levels have exceeded the preconsolidation stress and that the resulting subsidence may be permanent (inelastic). Continued monitoring in the southern Coachella Valley is warranted because ground-water levels continue to decline to record-low levels in some areas of the valley and, therefore, the significant amounts and increasing rates of land subsidence documented by this study are expected to continue.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5251
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